1. Field of the Invention
The present invention relates to a method and apparatus for correcting a signal transmitted via a communications channel having signal channels carrying base band components which are phase-shifted relative to each other.
2. Description of the Prior Art
In communication systems it is often desired to transmit multiple base band components per carrier frequency which are phase-shifted relative to each other.
Common such principles are denoted Phase Shift Keying, PSK, or Quadrature Amplitude Modulation, QAM, in which a carrier frequency is amplitude modulated with a first base band signal and a second base band signal phase shifted 90 degrees relative to the first base band signal. Typically, the first base band signal is denoted an in-phase component and the second base band signal is denoted a quadrature phase component.
In order to transmit and receive PSK signals with a sufficient high quality, transmitters and receivers using intermediate frequencies have been used so far. Intermediate frequencies are frequencies in the range between the base band and the carrier frequency. However, in systems and consumer products where cost and complexity are important parameters to minimise, this solution is far from optimal since it requires numerous components to generate the intermediate frequency.
Homodyne or direct conversion receivers are cheaper and less complex candidates for receiving PSK signals. These receivers are characterised by converting the frequency band about the carrier frequency direct down to base band and vice versa for transmitters. However, in homodyne or direct conversion receivers distortion in the form of amplitude and phase mismatch between the base band components are much more difficult to control.
In communication systems communicating digital symbols e.g. in the most common mobile telecommunications systems such as the Global System of Mobile telecommunications (GSM) the symbols are transmitted in bursts or packets. Such bursts or packets typically include sequence of training symbols in the burst or packet for the purpose of correcting symbols representing the payload i.e. the information that it is the purpose to communicate. Symbols are extracted by sampling the base band components.
The above mentioned communication systems find its application in mobile communication terminals or base stations in cellular communication systems e.g. TDMA systems comprising GSM and EDGE mobile telephone systems.
Thus there is a need for a communication method providing a high communication quality while using simple means for the communication.
U.S. Pat. No. 5,774,375 discloses a method and an apparatus for correcting signal-pairs from vectors that represent in-phase signals (I) and quadrature signals (Q). The correction includes calculating the most probable correction values of offsets from a center point in the IQ-plane of signal-pairs using a sequence of I and Q signal values and based on an equation of a circle. Amplitude and phase errors are corrected as a function of the sequence of I and Q signal values after a preceding correction of the center point.
U.S. Pat. No. 5,896,061 discloses a homodyne receiver and a method of correcting a received signal. There is provided an arithmetic unit that is designed for converting an ellipse set by distorted I and Q signals into a circle. The ellipse is determined by parameters using at least five samples of the I and Q signals. From the ellipse parameters the errors causing the elliptical form are then calculated and compensated.
U.S. Pat. No. 5,604,929 discloses a system for correcting gain and phase errors in a direct conversion receiver having a pair of signal channels carrying I and Q base band signal components in a quadrature relationship. The system operates by treating phase and amplitude errors as resident in the channel carrying Q baseband signal components and by generating a plurality of intermediary signals that incorporate a trigonometric product the I and Q components to produce a pair of correction factors that maintains a matched quadrature relationship between the two signal channels.
U.S. Pat. No. 5,095,533 discloses a direct conversion receiver having a tri-phase architecture including three separate base band signal channels. Two of the base band components are then used to form a highly accurate quadrature component which may be utilized in combination with the in-phase or reference component for demodulating the base band signals.
However, the prior art is only concerned with correcting phase and gain errors arising from imperfect phase shift means in the receivers/transmitters.
Consequently, the prior art involves the problem that distortion in the form of amplitude and phase mismatch are not corrected sufficiently for in the process of correcting a received signal.